The present invention relates to a spark plug having a novel center discharge electrode and a manufacturing process thereof.
In the prior art, a general conventional construction of the spark plug comprises a refractory insulator with the center bore, a terminal rod and a center discharge rod electrode, the latter two being inserted in the center bore of the refractory insulator and being electorically conductively sealed with a conductive sealing glass composition therein, and, if desired, a resistor material is incorporated in the center bore between the terminal rod and the center electrode.
A conventional type of the metallic center discharge electrode (center electrode hereinafter) is formed of a nickel or nickel alloy rod or such rod with a copper core. For those spark plugs, a high dimensional accuracy for each parts, particularly that for an inner diameter of a discharge end portion of a refractory insulator (insulator hereinafter) and for an outer diameter of the center electrode is required. Furthermore, a sealing process for these parts should be controlled in a highest accuracy in respect with sealing and impact resistance properties of such products. Those are marked as drawbacks of this type of the spark plug.
One type of center electrode has a welded noble metal tip at its discharge end which renders an improved durable life, whreras it requires an additional step of welding and a larger amount of discharge tip metal is consumed more than actually necessary for discharge purpose per se although merely a minor amount thereof functions as such in the practice. The discharge tip is usually welded to the head portion with an enlarged diameter of a base electrode rod formed in a rivet like (T type cross-section) form, which is necessary for preventing the discharge electrode from dropping off in order to mechanically form-lockingly secure it to a discharge end bore of the insulator, because a detriment in bonding strength of welded portion during an application under a high temperature would otherwise be caused. Those facts make this type also accompanied by a higher cost of manufacture.